1. Field of the Invention
This invention relates to head stack assemblies for supporting read/write heads adjacent rotating disks in disk drives and more particularly, to a swage mount attachment having a micro-hub for attaching a head suspension assembly to an actuator arm.
2. Description of the Prior Art
In hard disk drives data are stored on magnetizable surfaces of a plurality of rotating disks that are mounted in a coaxial stack on a housing of the drive. Transducer heads that write data to and read data from the disk surfaces are supported by an actuator that is mounted on the same housing and can be actuated to position the transducer heads in alignment with concentric data tracks defined on the disks. Each transducer head is attached to one end of a head suspension that is connected to an actuator arm that extends from the actuator body. Each suspension includes a flexible load beam constructed of light sheet steel that has a bend formed in it. The load beam acts as a spring that forces the head against the disk surface with an accurate load force or “gram load”. An air bearing caused by the rotating disks lifts the heads slightly off of the disks so that the heads fly at a specific height across the disk surfaces. The air bearing force is counteracted by the suspension gram load.
The head suspension is attached to an actuator arm using a swage mount that forms a part of the head suspension. The swage mount includes a flat flange portion and a cylindrical hub portion or boss. The swage mount hub is passed through a load beam clearance hole and the flange is spot welded to the load beam. Alternatively, the hub and load beam clearance hole are aligned and the flange is welded on the opposite side. The combined swage mount, load beam and a flexure make up the head suspension, and the suspension has the hub of the swage mount extending beyond the load beam and concentric with the clearance hole.
The hubs of the suspensions are inserted into actuator arm holes formed through actuator arms extending from an actuator body. In the middle actuator arms, the hubs of two suspensions enter the arm boss hole from each end of the hole, so that the transducer heads of the suspensions face in opposing directions. A swage ball is passed through the concentric cylindrical hubs to force the peripheries of the hubs to expand (swage) into tight interference engagement with the inner peripheries of the actuator arm holes.
Problems with this method of mounting transducer heads have arisen as the need for increased data storage capacity in hard disk drives has grown and the size of the disk drive has decreased to fit in small lap top computers. The problem of forming a strong connection between the actuator arms and the transducer suspensions has been made more difficult as the size of the components has become smaller. In the prior art, relatively high swaging forces are needed to insure that a swage mount makes a strong connection with the actuator arm boss hole. As the parts get smaller and thinner, these high forces cause unacceptable large distortions in the flange portion of the swage mount which then distort the load beam and cause gram load changes. These distortions can also adversely affect the resonance characteristics of the assembly such that the head does not stay on track during reading and writing operations.
One method for reducing the overall drive size is to reduce the size of the stacked vertical joint connecting the load beam to the actuator assembly. For example, in U.S. Pat. Nos. 6,183,841 and 5,689,389, a low profile swage mount fastener is used to connect a load beam to an actuator arm of an actuator assembly. Because the swage mount fastener has a low profile, the overall height of the disc drive may be reduced. However, a disadvantage of using a low profile swage mount fastener is that as performance demands increase, a low profile swage mount fastener may provide less torque retention than is required to withstand the forces on the load beam.
In U.S. Pat. Nos. 6,183,841 and 5,689,389 the torque retention characteristics of a low profile swage mount fastener were increased by modifying the internal geometry of the swage mount. However, the level of torque retention that can be achieved solely by modifying the swage mount design is limited. Without increased torque retention values, the acceleration rate a load beam can withstand is limited, which imposes an upper limit on the speed at which the read/write head can be positioned. This in turn will limit the overall access time a disc drive can achieve, a key parameter of disc drive performance.
U.S. Pat. Nos. 6,231,689 and 6,351,349 are directed at overcoming shortcomings of the prior art. Each patent provides a surface hardened swage mount, a method of hardening the metal from which such parts are made, and for connecting a disc drive actuator arm to a load beam, which results in an increased torque retention characteristic of the swage mount.
As described above, as swage mounts get smaller to accommodate the geometries of smaller disk drives, reduced retention torque becomes a problem and this has given rise to a need to increase retention torque. The need to increase retention torque is a problem especially with nickel-plated aluminum actuator arms. A swage mount is disclosed in co-pending application Ser. No. 10/037,643 “Surface Hardened Swage Mounts for Improved Performance” of Ernest Swayney and Steve Braunheim (incorporated herein by reference) in which the outer surface of the hub includes numerous protrusions that are less than approximately 50 microns in height. The protrusions are primarily comprised of a material (such as a carbide or a nitride) which is different from the stainless steel hub. Preferably, the protrusions are substantially harder (such as at least 50 hardness Vickers harder) than the base material of the hub. The purpose of the protrusions is to provide greater torque retention when the swage mount is swaged to an actuator arm.
During manufacture, chromium carbide or chromium nitride is precipitated out of a base metal onto the outer surface of the hub resulting in the surface protrusions. The surface protrusions stick out of the hub outer surface and grab into the aluminum actuator arm boss hole when the hub is swaged. These and other methods of creating hardened modules on the outer hub surface can boost retention torque by 60%–100%.
Swage mounts containing carbides provide higher retention torque than nitrided parts, but tend to shed a higher volume of particles from the surface. Due to the present emphasis on cleanliness within the industry, this currently limits the use of carbides, the most effective precipitate.
During current manufacturing of swage mounts, the swage mounts are subjected to processes that remove burrs, which may include tumbling using porcelain beads. The reason the swage mount is deburred is that a burr may flake off and contaminate the drive mechanism. Furthermore, a burr can cause the swage mount to stand off and not mate with the load beam properly. Tumbling to deburr the swage mount using porcelain beads that are predominately aluminum oxide may result in aluminum oxide particles coming loose and becoming embedded in the surface of the disk. Studies of failed disk drives have shown that aluminum oxide separating from the beads has been found on the disk surface at the site of a head crash. Even if the head does not crash, an aluminum oxide particle embedded on the disk can cause a thermal asperity. As the head passes over the particle, the head may be damaged by heat from the friction or an inaccurate reading may occur.
Manufacture of the material used to fabricate the swage mounts, typically stainless steel, often results in the introduction of metal oxides such as alumina and magnesia into the melt. These oxides and other contaminates in the base metal can form inclusions that may potentially be exposed at the surface of the swage mount after manufacturing. These inclusions, should they become loose and fall from the base material, can pose a threat to drive operation in the form of a head crash or thermal asperity, as described above.
Copending application Ser. No. 10/241,609 of Damon D. Brink, et al “Plated Base Plate For Suspension Assembly In Hard Disk Drive” (incorporated herein by reference) discloses a method to cover the imbedded particles and material inclusions, to prevent them from coming loose from the swage mounts during service. The hub is plated with metal to improve the cleanliness and retention torque of the swage mounts. When applicable, the metal plating is used to prevent the protrusions from separating from the hub and contaminating an assembled disk drive. The plating deposit may include, but is not limited to, Ni, Cr, Pt, Pd, Rh, Au, and Ag, or combinations or layers thereof. The hub outer surface prior to plating may be provided with surface protrusions that increase torque retention when the hub is swaged. In all swage mounts, with or without surface protrusions, the microstructure and associated tribological characteristics of the metal plating is such that retention torque is increased.
It is desirable to provide an optimum swage mount geometry in which the gram load and resonance changes inherent in swaging are reduced and a large retention torque is created even in low hub height configurations that offer limited retention torque in a standard hub geometry.
With the above methods of increasing retention torque now available it is therefore possible and desirable to provide a swage mount that has a smaller hub than a conventional swage mount, a torque retention capability comparable to the prior art and a reduced pre-load change caused by the swaging process.